In a headlight mounted on a vehicle, a bright discharge lamp or an LED (light emitting diode) for brightly lighting in any direction, in place of a conventional halogen bulb as a light source, is widely used to enhance safety during night driving, while having high design qualities and an upscale image.
In mounting the aforementioned bright light source on the vehicle, when a rear portion of the vehicle is lowered and inclined, in other words, when a front portion of the vehicle is raised and the light emitting direction of headlights is inclined upward due to, for example, the presence of a person who takes a rear seat of the vehicle or baggage loaded in a trunk, it is necessary to lower the light emitting direction of the headlights, that is, the optical axes of the headlights and keep constant the optical axes with respect to a road surface so as not to dazzle a driver in an oncoming vehicle, and so as not to give an unpleasant feeling to pedestrians facing the headlights. To put it briefly, as for the vehicle having the bright light source, it is essential to mount a headlight optical axis control apparatus which lowers the light emitting direction of the headlights and returns the light emitting direction of the headlights to a direction prior to change when the vehicle is inclined and the light emitting direction of the headlights is varied upward due to at least the presence of a person who rides on the vehicle or the baggage loaded in the trunk.
Note that a person rides on a vehicle or the baggage is loaded on the vehicle during stoppage of the vehicle, and optical axis control during stoppage of the vehicle is main control performed by the headlight optical axis control apparatus.
Incidentally, the control of the headlight optical axis is one wherein the optical axis is operated up and down, in such a way as to cancel a change in an inclination angle (hereinafter referred to as “vehicle angle”) of the vehicle with respect to the road surface in order to return the light emitting direction of the headlights to an original direction when the vehicle is inclined in an up-and-down direction as described above, so that, first, it is necessary to measure the vehicle angle.
Conventionally, the shrinking amounts of front and rear suspensions of a vehicle, i.e., the sinking amounts of front and rear axle portions have been measured by using stroke sensors mounted on the front and rear suspensions (suspension devices) of the vehicle, and the vehicle angle has been calculated on the basis of a difference between the sinking amounts of the front and rear axle portions and a length of a wheel base.
In recent years, instead of a configuration using the stroke sensors mounted on the suspensions, a configuration using an acceleration sensor that can detect gravitational acceleration has been studied as disclosed in, for example, Patent Literature 1. In the configuration using the acceleration sensor, it is easy to detect a change in the inclination angle of a vehicle that is being stopped, and it is easy to obtain the vehicle angle at the present time by accumulating variation amounts caused by the boarding or alighting of persons with respect to an initial vehicle angle. On the other hand, an offset, which varies with time, is included in output of the acceleration sensor, and the vehicle angle obtained by accumulating the variation amounts potentially includes a cumulative error, so that there is a problem that the accuracy of the vehicle angle obtained by accumulating measured values and the change is low. Accordingly, in order to maintain the headlight optical axis stably at a correct angle for a long period of time, it is necessary to ensure the accuracy of the vehicle angle by adding some correction to the acceleration measured by the acceleration sensor or removing the cumulative error from the vehicle angle.
An optical axis control apparatus disclosed in Patent Literature 1 enhances the accuracy of the vehicle angle while using an acceleration sensor in two axes in the front-and-rear direction and up-and-down direction of the vehicle, and the optical axis control apparatus measures the acceleration and performs the optical axis control during the travelling of the vehicle, in order to perform preferable headlight optical axis control, besides the optical axis control during the stoppage of the vehicle. By using the acceleration measured during the travelling of the vehicle, the optical axis control apparatus disclosed in Patent Literature 1 obtains a changing direction of the acceleration at each time or obtains a changing direction of the acceleration on the basis of two acceleration measured at different measuring timing, calculates the vehicle angle, and controls the optical axis on the basis of a change in the vehicle angle.